This invention relates to a power steering apparatus for an automobile or other vehicle in which the force required to turn the steering wheel of the vehicle is lessened by the operation of an electric motor.
In a power steering apparatus for an automobile, when the driver applies torque to the steering wheel of the automobile, the steering shaft is caused to undergo an initial slight rotation. The rotation is detected by a rotation detecting mechanism, and a motor is energized in response to this initial rotation to rotate the steering shaft and decrease the force which must be exerted by the driver.
Various arrangements have been proposed for connecting a motor to a steering shaft so as to rotate the steering shaft when the motor is operated. In one type of conventional apparatus, a motor for providing power to a steering shaft drives a reduction gear through a clutch. The output shaft of the reduction gear has a pulley mounted thereon, and rotational force is transmitted from this pulley to another pulley mounted on the steering shaft via a V-belt.
With such a structure, the power steering mechanism becomes extremely bulky, since the reduction gear, the clutch, and the motor are all separate units independent of the steering shaft. Furthermore, although the motor can be disconnected from the reduction gear via the clutch, the steering shaft is at all times connected to the reduction gear by the V-belt and the pulleys. Accordingly, if the motor fails and the steering shaft must be rotated manually by the driver, the driver must exert a force sufficient not only to turn the wheels of the vehicle but also to overcome the resistance of the reduction gear which turns with the steering shaft. Therefore, steering becomes difficult.
According to another arrangement, a motor for decreasing the steering force is connected not to the steering shaft but to the steering gear rack of the vehicle. For example, in Japanese Laid Open Utility Model Application No. 57-120164, a steering shaft and a motor are drivingly connected to a steering gear rack by separate pinions. When either the motor or the steering shaft is rotated, the pinions rotate, causing the steering gear rack to translate, thereby changing the direction in which the wheels of the vehicle point. However, no clutch is provided between the motor and the steering gear rack, with the result that when the motor fails and it is necessary for the driver to move the steering gear rack manually, the driver must apply enough force not only to move the steering gear rack but also to rotate the motor Accordingly, this arrangement, too, has the disadvantage that steering becomes difficult.